Thread rolling and rolled threaded objects



Dec. 2, 1969 o c; WILKINS 3,481,178

THREAD ROLLING AND ROLLED THREADED OBJECTS Filed April 10, 1967 ,2 Sheets-Sheet 1 PA R T PROM/C50 24 0/5 mooucso EY CUTTING T001.

INVENTOR. ORE/V C. W/l. K/NS ATTOR/VJFYS.

1969 o. c. WILKINS THREAD ROLLING AND ROLLED THREADED OBJECTS 2 Sheets-Sheet 2 Filed April 10, 1967 PART BEING PRODUCED PART DEING PRODUCED DY DIE PART BEING PRODUCED BY DIE 6 MW 5 NK h Wu 5 WW m 8/0 c w m A m w M m o 6 M m PART United States Patent 3,481,178 THREAD ROLLING AND ROLLED THREADED OBJECTS Oren C. Wilkins, Redondo Beach, 'Calif., assignor to Hi- Shear Corporation, Torrance, Calif., a corporation of California Filed Apr. 10, 1967, Ser. No. 629,736 Int. Cl. B21d 17/04 U.S. Cl. 72-469 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to thread-rolling, and particularly to the production of improved threads on an object secured by the thread-rolling process, examples being threaded shanks on bolts and studs.

According to the present invention a unique roll die is formed by cutting protrusions on the die, and the die produced may be either in the round or in the fiat form. In the round form the protrusions are helical and in the flat form they are parallel, substantially straight, and slanted. The protrusions in the die are formed with first protrusion means disposed in a nominal face having a generator parallel to a die axis. The first protrusion means has leading and lagging flanks. Second protrusion means is also included, which is cut into the die and is disposed in a nominal face oblique to the die axis. The second protrusion means also has leading and lagging flanks, and they are parallel, respectively, to the leading and lagging flanks of the first protrusion means. Thus, the first protrusion means is disposed generally parallel to the second protrusion means, and the second protrusion means recedes from the first protrusion means and toward the axis, but both are compatible with a thread which is formed by both of them.

A threaded object formed by reaction with this die will have a group of thread convolutions fully compatible with a load-bearing region of another thread-bearing object, and a group of transition threads of full form but reducing in depth which are also compatible with the other thread-bearing object. The transition threads have no sharp transition regions, and are more resistant to cyclical, fatigue forces.

This invention relates to improvements in thread rolling and in rolled threads.

The art of rolling threads onto objects such as bolts, screws and studs is well known. Basically, the process comprises rolling an object against a die of proper contour whereby a helical thread is formed on the object.

As this process is usually performed, a preformed blank is used which has the general configuration of the ultimate product except that it has a round shank without threads. Threads are formed on the object by pressing a die against the object. The face of this die is the obverse of the desired threads.

There are several advantages in the formation of threads by thread rolling that are not achieved by thread cutting, and these advantages reside in both economy of manufacture and improved strength in the product. For example, because threads of full form may be formed by a single pass of the die, the rolling process is faster than cutting. Also, since a conventional cutting process requires removal of part of the metal of the blank, metal is wasted in the process of cutting the thread. Furthermore, the rolling process cold-works the metal of the blank, and produces stronger threads than those which are formed by cutting. It is evident that rolled threads have considerable advantages. However, they have serious shortcomings When manufactured by prior art techniques.

3,481,178 Patented Dec. 2, 1969 "ice One such shortcoming is that the thread closest to the head of a screw or bolt has a knife-edge, or sharp, partially formed thread, at its end. This knife-edge por tion results from an abrupt ending of the thread in the metal of the blank. This thread is not a fully formed thread, and is a sharp discontinuity. It therefore is a weak point and if a region of failure exists in the thread, it is usually expected here. A more reliable bolt would be one whose last thread fairs into the metal, leaving no sharp discontinuities.

It is an object of the present invention to provide apparatus for the production of threads by thread rolling wherein the threads fair into the metal of the threaded object, leaving no knife-edge portion.

A roll die according to this invention is formed by cutting protrusions on a die; the die may be either a flat or a round roll die. It is to be understood that the term roll die means dies which are to be used in. rolling processes, and includes both flat and round roll dies. The protrusions in the die include first protrusion means dis posed in a nominal face having a generator parallel to a die axis, and second protrusion means disposed in a nominal face having a generator oblique to the die axis. The first and second protrusion means each have leading and lagging flanks, all the leading flanks being parallel to each other and all the lagging flanks being parallel to each other. Thus, the first protrusion means is disposed generally parallel to the second protrusion means, but the second protrusion means reduces in depth from the first protrusion means and toward the axis. Both protrusion means are compatible with a thread to be formed by both of them.

The apparatus is useful for forming a transition between threads of difierent diameters. The thread configuration developed by the present apparatus may be any desirable configuration, for example, conical, cylindrical or ogival. The apparatus is also useful for forming tapered start threads for threaded objects, such as studs and pipes. Also, multiple start threads of full form may be formed by a single pass of the die.

Another object of the present invention is to provide a threaded object having threads formed by rolling a surface of a blank against a roll die, which threads are generally disposed normal to the axis of the blank in a configuration having a portion oblique to the axis of the blank.

Another object of the present invention is to provide a configuration of protrusions on a roll die, which configuration has a portion oblique to a reference axis of the die, and which protrusions are generally disposed normal to that axis.

Another object of the present invention is to provide a method for forming a configuration of protrusions on a roll die, which configuration has a portion oblique to a reference axis of the die, and which protrusions are generally disposed normal to that axis.

A threaded object according to this invention is formed by reaction with the die according to this invention, and will have a group of thread convolutions fully compatible with the load-bearing region of another thread-bearing object. A group of transition threads having full form but reducing in depth are also formed on the threaded object, and are also compatible with the load-bearing region of the other thread-bearing object to which it is intended to be joined. Since the transition threads are reducing in depth, they have no sharp discontinuities and are more resistant to fatigue loads than heretofore known.

According to an optional and desirable feature of the threaded object according to the present invention, the threaded object is formed from a blank having a first nominal cylindrical face parallel to an axis. A second nominal frusto-conical face, adjacent said first nominal face, is oblique to said axis at a first angle. Threads are rolled into said first nominal face, which threads are generally disposed normal to said axis and have a uniform pitch and depth. Threads are simultaneously rolled into said second nominal face, which threads are likewise generally disposed normal to the axis and parallel to the threads in the first nominal face, but are arranged in a configuration which is oblique to said axis at an angle greater than said first angle. The threads thus formed in said second nominal face have a successively smaller depth into the blank.

The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which:

FIGS. 1 and 2, respectively, are fragmentary cutaway top and side views of a cutting tool in the process of forming configurations of grooves in a roll die in accord ance with the present invention; FIG. 1 being taken along line 11 of FIG. 2;

FIG. 3 is a perspective view of the presently preferred embodiment of a flat roll die according to the present invention having a configuration of grooves being formed in accordance with the method of the present invention;

FIG. 4 is a side elevation of the presently preferred embodiment of a threaded object having threads which have been rolled into the surface thereof with a die according to the present invention;

FIG. 5 is an enlarged cutaway side view of a portion of the threaded object illustrated in FIG. 4, having selected threads being formed by a die according to the present invention;

FIGS. 6 and 7 are top and side views, respectively, of the method of forming threads in an object with a flat roll die; and

FIGS. 8 and 9 are top and side views, respectively, of the method of forming threads in an object with a round roll die.

Referring to FIGS. 1 and 2 there is shown a cutting tool 10 having an axis 12. Cutting tool 10 has a nominal face, indicated by dashed lines 14, which is oblique to axis 12 at angle 0. Cutting teeth 16 protrude from face 14 and are equally spaced apart. Each tooth 16 has a first leading cutting face 18, a second leading cutting face 20 and a lagging cutting face 22.

As best illustrated in FIG. 2, cutting faces 18 and 22 are preferably at substantially equal angles from reference axis 12 so as to form a groove between them which is generally normal to axis 12. The depths of cutting faces 18 and 22 toward axis 16 for each groove are preferably equal, and cutting face 20 forms a transition be tween cutting face 18 and the apex of the cutting tooth. Thus, the grooves between each successive cutting tooth 16 are disposed generally normal to axis 12.

A roll die 24 is illustrated in FIGS. 1 and 2 having a plurality of protrusions 26 normal to an axis or reference plane 28. Protrusions 26 may be formed in any convenient manner such as by cutting them. Protrusions 26 have lead flanks 27 and lag flanks 29 with each protrusion 26 formed between them. The crests of protrusions 26 are all equidistant from axis or reference plane 28.

When cutting protrusions 30 in roll die 24, axis or reference plane 28 of the roll die is aligned parallel to axis 12 of the cutting tool. The pitch between cutting teeth 16 of the cutting tool, as projected along axis 12 or 28, is equal to the pitch between successive protrusions 26 of die 24. Cutting tool 10 is moved relative to the roll die to cut a plurality of protrusions 30 in the roll die. As shown in FIG. 2, a protrusion 30* has been formed in die 24 by cutting teeth 32 and 34 of the cutting tool. Protrusions 30 have a pitch, as projected along axis 28, equal to protrusions 26 of die 24. Flank 31, formed by cutting face 18 of the tool, is parallel to flanks 27 of protrusions 26; and flank 33, formed by cutting face 22, is parallel to flanks 29 of protrusions 26. Therefore, protrusions 30 are generally parallel to protrusions 26 and are preferably generally disposed normal to axis 28 of the roll die. Each succeeding protrusion 30 is located closer to axis or reference plane 28 as it recedes from protrusions 26. Protrusions 30 are formed in nominal face 38 which is oblique to the axis or reference plane 27 by a configuration angle 0.

As illustrated in FIG. 1, cutting tool 10 is moved in a direction having a component in the C direction paral lel with axis 12, and having another component in the A direction normal to axis 12. The movement in the AC plane shown in FIG. 1 is in a direction displaced from the A axis by an angle (sometimes referred to as the helical angle), and each protrusion formed in die 24 is parallel to the direction of the moving of the cutting tool in the AC plane defined by angle The term helical angle is applied to angle of the roll die because angle 5 of the protrusions later determines the angle of the helix of the threads which are rolled by the roll die.

As illustrated in FIG. 2, cutting tool 10 is also moved in a direction having a component in the B direction, normal to. both the A and C directions. The movement in the B-C plane shown in FIG. 2 is in a direction displaced from the C axis by an angle 0 (sometimes hereinafter referred to as the configuration angle), and each protrusion cut in die 24 by cutting tool 10 is normal to axis 12 of the cutting tool and at a successively different height determined by the configuration angle 0. Angle 0, therefore, determines the configuration of protrusions 30 on the die.

Protrusions 26 of die 24 extend generally normal to axis 28 in nominal face 36, parallel to axis 28, and protrusions 30 extend generally normal to axis 28 in nominal face 38, oblique to axis 28 at angle 0. Protrusions 28 and 30 have equal angles In FIG. 3 there is illustrated a flat roll die 24 having a plurality of protrusions being formed by cutting tools 10 and 11. Although FIG. 3 illustrates the die being cut by two cutting tools, it is understood that the cutting operation of each cutting tool may be independent of the other and that the tools may be used separately in subsequent cutting operations or together in a single, unitary cutting operation.

Cutting tool 11 is rotated about its axis 13, which is parallel to axis 28 of the die during a cutting operation. Cutting tool 11 is moved in the AC plane in a direction displaced from the A direction as determined by angle Cutting tool 11 cuts into the die to form a plurality of protrusions 26 in a face parallel to the reference axis or plane 28. Protrusions 26 are flanked by leading and lagging flanks so that the leading flanks are parallel to each other and the lagging flanks are parallel to each other.

Cutting tool 10 is rotated about its axis 12, which is parallel to axis 28 of the die during a cutting operation. Cutting tool 10 is moved in a direction displaced from the A direction in the AC plane by angle 4; and in a direction displaced from the C direction in the BC plane by configuration angle 0. The movement in the B direction determined by the configuration angle is necessary to provide a smooth transition between protrusions 26 and 30 at their junction. Cutting tool 10 cuts into the die to form a plurality of protrusions 30 in one, or several, passes in a face oblique to the reference axis or plane 28 by a configuration angle 6, determined by the face of the cutting tool. Protrusions 30 are flanked by leading and lagging flanks so that the leading flanks are parallel to each other and to the leading flanks of protrusions 26, and so that the lagging flanks are parallel to each other and to the lagging flanks of protrusions 26. The direction of movement in space of cutting tool 10 is determined by the equation:

1 tan cot 0 where B and A are distances of relative movement in the B and A directions, respectively.

The die formed by cutting protrusions with tools and 11 has a plurality of protrusions 26 in a face 36 having a generator parallel to axis 28 and a plurality of protrusions 30 in a face 38 oblique to axis 28. Protrusions 26 and 30 are disposed generally normal to each other in the sense that their respective leading and lagging flanks are parallel. Although the present disclosure describes in detail a flat roll die, it is understood that a round roll die may be formed embodying the present concept by merely revolving the A direction about a circular path conforming to the diameter of a round roll die. Such a round roll die will have a similar protrusion configuration angle 0 relative to its axis. FIGS. 8 and 9, to be described in detail below, illustrate a round roll die in accordance with the present invention.

It is to be understood that the configuration of threads oblique from the axis of the blankmay be any desirable configuration, depending upon the configuration of the protrusions in the production roll die. Thus, by altering the die configuration angle 0 through several angles, a die configuration may result for producing conical, ogival and/ or cylindrical threads.

When a blank object, having the general shape of a threaded object, but without threads, is pressed and rolled against nominal faces 36 and 38, helicalthreads are formed at the desired helical angle, Transition threads are also formed by the protrusions in nominal face 38, which threads are of full form, but each convolution is of less depth in the surface, and the threads appear to fair into the metal of the blank. This is due to the fact that g the crest diameter of the transition threads decreases, 'while the root diameter of the transition threads increases.

FIG. 4 illustrates a screw 54 formed by thread rolling in accordance with the present invention, and FIG. 5 is an enlarged cutaway side view of the pertinent threads of screw 54. Axis 52 of blank or screw 54 is aligned parallel to axis 28 of die 24. Screw or blank 54 has a configuration similar to the finished object except that it has no threads. Blank 54 has a cylindrical surface 56 disposed about axis 52 and preferably has a frustoconical face 58 joining cylindrical face 56 and receding therefrom in a direction from axis 52 determined by angle a.

Protrusions 26 press into surface 56 of the blank, forming the roots of threads 60 by displacement of metal. The metal so displaced is forced intothe spaces between protrusions 26 to form the crests of threads 60. Thus, by adjusting the pressure between the die and the blank, protrusions 26 will displace enough metal away from the roots of the threads being formed and into the region above the surface of the blank to form the crests of the threads in a single pass. Planks 59 and 61 of threads 60 are formed by flanks 27 and 29, respectively, of the roll die.

Protrusions 30, protruding from surface 38 of the die and having a depth configuration determined by angle 0 from the die axis, do not protrude as d'eeply'into the surface of blank 54 as do protrusions 2'6..As a result, metal displaced by protrusions 30 in blank 54 does not entirely fill the space between each of the successive protrusions 30. Threads 62 and 64, formed by protrusions 30, have root diameters which are successively greater. The crests of threads 62 and 64 preferably do not exceed the crest diameter of threads 60. Planks 63 and 65 of the receding threads are formed by flanks 31 and 33, respectively, of the roll die, and are therefore parallel to flanks 59 and 61, respectively, of threads 60. Threads 62 and 64 have an identical axis pitch as threads 60, and are disposed generally normal to axis 52 of the blank, but each succeeding transition thread is not as deeply formed into the blank as the immediately preceding thread.

The configuration of the crests of the receding threads 62 and 64 recedes towards axis 52 at an angle to. This is due to the fact that thread 62, being the most fully formed thread of the receding thread configuration, is formed by the largest displacement of metal in the surface of blank 54. Each successive thread is formed by the displacement of less metal, thereby forming the crest configuration angle at of the receding threads. The threads are of successively less depth into the blank, thereby fairing into the metal. Thus, no knife-edge thread having a sharp discontinuity is formed.

As illustrated in FIGS. 4 and 5, blank 54 has a frustoconical face 58 receding from the axis 52 at an angle at and joined to cylindrical face 56 of the blank. Such a frusto-conical face as 58 is, of course, optional but has been found especially desirable in situations where the receding thread configuration should have two or more transition threads, or where an enlarged shank is desired for the threaded object 54.

Frusto-conical face 58 recedes from cylindrical face 56 in a direction from axis 52 determined by angle a. Protrusions 26 of die 24 form full-form cylindrical threads 60 in face 56 of the blank, and protrusions 30- form receding threads 62 and 64 of full form in frusto-conical face 58. The configuration angle 9 of threads 62 and 64 is greater than angle a of face 58 so that the threads formed in face 58 recede in depth along face 58. The resulting threads on blank 54 are therefore cylindrical on face 56 and recede conically on face 58. The threads formed in face 58 therefore fade into the surface of the blank, leaving no knife-edge or sharply discontinuous thread.

FIGS. 6 and 7 illustrate the production of a threaded object with a flat roll die according to the present invention. A pair of fiat roll dies 70 and 72 similar to that illustrated in FIG. 3 are pressed against a surface 74 of blank 76 in a position so that axes 71 and 73 of the roll dies are parallel to axis 75 of the blank. Dies 70 and 72 are then moved in a direction perpendicular to their axes in directions indicated by arrows 77 and 78, and blank 76 rotates about its axis in a direction indicated by arrow 79. Surface 74 of the blank thus rolls along the die and the resulting product is similar to that illustrated in FIG. 4.

FIGS. 8 and 9 illustrate the production of a similar product with round roll dies. Dies 80 and 82 are pressed against surface 84 of blank 86 in a position so that axes 81 and 83 are parallel to axis of the blank. Dies 80 and 82 are rotated about their axes in directions indicated by arrows 87 and 88 and blank 86 rotates about its axis in the direction indicated by arrow 89. Surface 84' of the blank thus rolls along the dies and the resulting thread configuration illustrated in FIGS. 4 and 5 is thus formed on surface 84 of blank 86.

The present invention provides a new and useful roll die and method of forming a configuration of protrusions on a roll die, which die forms a thread configuration on a blank when rolled against the blank. The threads formed on the blank fair into the metal of the blank, leaving no knife-edge or sharply discontinuous thread. The thread which fairs into the metal has a shear strength at least equal to the other threads, thereby reducing thread strippmg.

This invention is not to 'be limited by the embodiment shown in the drawing and described in the description, which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.

What is claimed:

1. A roll die for rolling threads into a blank, said blank having an axis about which the blank is to be rotated during a thread rolling operation, said die having an axis, the axis of said blank being parallel to the axis of said die during a thread rolling operation, said roll die comprising: first protrusion means for rolling threads into said blank, said first protrusion means having first flank means on one side thereof and second flank means on the opposite side thereof, said first and second flank means defining said first protrusion means between them, said first protrusion means having first crest means and first root means, said first crest means defining a first nominal face on said roll die having a first generator substantially parallel to said die axis, and said first root means defining a second nominal face on said roll die having a second generator substantially parallel to said first generator and spaced therefrom; second protrusion means for rolling threads into said blank, said second protrusion means having third flank means on one side thereof and fourth flank means on the opposite side thereof, said third and fourth flank means defining said second protrusions between them, said third flank means being substantially parallel to said first flank means and said fourth flank means being substantially parallel to said second flank means, said second protrusion means having second crest means and second root means, said second crest means defining a third nominal face on said roll die having a third generator oblique to said die axis at an angle, and said second root means defining a fourth nominal face on said roll die having a fourth generator substantially parallel to said third generator and spaced therefrom; said first and third nominal faces being joined at a first junction and said second and fourth nominal faces being joined at a second junction, the third generator sloping away from the first generator and towards said die axis from said first junction and the fourth generator sloping away from the second generator and towards said die axis from said second junction; the distance between said third and fourth generators along a line substantially perpendicular to said die axis being substantially equal to the distance between said first and second generators along a line substantially perpendicular to said die axis, whereby the crest means and root means of said second protrusion means recede along the respective third and fourth nominal faces from the first and second junctions at said angle and the distance between the crest means and root means of said second protrusion means is not less than the distance between the crest means and root means of said first protrusion means.

2. A roll die according to claim 1 in which the roll die is a flat roll die and wherein said first protrusion means comprises a first plurality of protrusions having a uniform pitch, and said second protrusion means comprises a second plurality of protrusions having a uniform pitch, the crests of said first plurality of protrusions being joined to the crests of said second plurality of protrusions at said first junction and the roots of said first plurality of protrusions being joined to the roots of said second plurality of protrusions at said second junction, the crests of said first plurality of protrusions being at a uniform distance from said die axis and the crests of said second plurality of protrusions being at successively different distances from said die axis as said second plurality of protrusions recide from the junction between said first and third nominal faces along said die axis on said third nominal face.

3. A roll die according to claim 2 wherein the pitch of said first plurality of protrusions is equal to the projected pitch along the die axis of said second plurality of protrusions.

4. A roll die according to claim 1 in which the roll die is a round roll die and wherein said first and second nominal faces are cylindrical about said axis and said third and fourth nominal faces are frusto-conical about said axis, said first protrusion means comprising a first helical protrusion having a plurality of convolutions about said die axis, and said second protrusion means comprising a second helical protrusion having a plurality of convolutions about said die axis, the crest of said first helical protrusion being joined to the crest of said second helical protrusion at said first junction and the root of said first helical protrusion being joined to the root of said second helical protrusion at said second junction, the crest of each convolution of said first helical protrusion being at a uniform diameter about said die axis and the crest of each convolution of said second helical protrusion being at successively different diameters about said die axis as said second helical protrusion recedes from the junction between said first and third nominal faces along said die axis on said third nominal face.

5. A roll die according to claim 4 wherein the pitch of each successive convolution of said first helical protrusion is equal to the projected pitch along said die axis of each successive convolution of said second helical protrusion.

6. A threaded object having an axis and a shank and having convolutions of threads formed by pressing and rolling a surface of a blank against a roll die, said convolutions of threads including a first thread portion adjacent one end of the shank, each convolution of said first thread portion having substantially equal first root diameters and substantially equal first crest diameters, and a second thread portion, each convolution of said second thread portion having second root diameters and second crest diameters, one of said second root and crest diameters continuously increasing along said axis and the other of said second root and crest diameters continuously decreasing along said axis in such a manner that the second thread portion fairs into said shank remote from the threaded end of the object.

7. A threaded object according to claim 6 wherein the threads are exterior threads on the object and the second root diameter continuously increases while the second crest diameter continuously decreases.

8. A threaded object according to claim 6 wherein said surface includes a cylindrical portion and a frusto-conical portion, said first threads being formed in said cylindrical portion and said second threads being formed in said frusto-conical portion, the root diameters of said second threads lying in a nominal plane forming an acute configuration angle oblique to the axis, the frusto-conical portion being oblique to the axis at a first acute angle, said configuration angle being greater than said first angle.

9. A threaded object according to claim 8 wherein the crest diameters of the second threads lie in a nominal plane forming a second acute angle oblique to the axis, the nominal plane defining the second angle forming an obtuse angle with the nominal plane forming the configuration angle and forming an obtuse angle with the frusto-conical portion of said surface.

10. A threaded object according to claim 6 wherein said surface is revolved about said axis and is at least partially cylindrical, said first threads having first and second flanks, said first threads having a uniform pitch, said second threads having third and fourth flanks, the first flanks of said first threads being parallel to the third flanks of said second threads and the second flanks of said first threads being parallel to the fourth flanks of said second threads, said second threads having a uniform axial pitch, said second threads having thread configuration oblique to said axis, whereby said second threads are of successively decreasing depth into said surface.

11. A threaded object according to claim 10 wherein the projected pitch along said axis of said second threads is equal to the pitch of said first threads.

12. A threaded object according to claim 10 wherein said surface has a first cylindrical face parallel to said axis and a second frustoconical face oblique to said axis at a first angle, said first and second faces being joined at a junction, said second threads being rolled into said second face, said thread configuration being oblique from said axis at a second angle greater than said first angle, whereby said second threads are of successively decreasing depth into said second face.

13. A threaded object according to claim 12 wherein said first and second faces are outer faces of said object, said second face having a diameter which increases from the junction of said first and second faces, said second threads having a root diameter which increases at a rate greater than the rate of increase of the diameter of said second face.

9 14. A threaded object according to claim 13 wherein 2,314,391 the projected pitch along said axis of said second threads 2,896,484 is equal to the pitch of said first threads. 3,246,556

References Cited UNITED STATES PATENTS 808,165

440,330 11/1890 Rogers 72469 1,414,641 5/1922 Heames 72469 2,183,688 12/1939 Olson 7288 2,183,689 12/1939 Olson 7246-9 10 2,293,930 8/1942 Braendel 7288 X 8 18 10 3/1943 De Vellier 1010 X 7/ 1959 Hanna 72469 4/1966 Phipard 7288 X FOREIGN PATENTS 1/1959 Great Britain.

M ILTON S. MEHR, Primary Examiner US. Cl. X.R. 

